Report No. NADC 88097-60

L

EVALUATION OF THE MECHANICAL PROPERTIESOF 2091 AND 8090 ALLOYS

Lf) M. E. DONNELLANWO Air Vehicle and Crew System Technology Department

NAVAL AIR DEVELOPMENT CENTERWarminster, Pennsylvania 18974

(0N SEPTEMBER 1988

QFINAL REPORT

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11. TITLE (Include Security Classification)

EVALUATION OF THE MECHANICAL PROPERTIES OF 20811 AND 8090 ALLOYS

12 PERSONAL AUTHOR(S)MARY E. DONNELLAN

13a. TYPE OF REPORT 13b TIME COVERED 14. DATE OF REPORT (Year, Month,.Day) 15 PAGE COUNTFINAL IFROM 1 Feb87_ TO 30SOP 88 21 SEP 88 27

16 SUPPLEMENTARY NOTATION

17 COSATi CODES 18 SUBJECT TERMS (Continue on reverse of necessary and identify by block number)FIELD GROUP SUB-GROUP 2091 ALUMINUM TENSILE PROPERTIES

016 8090 ALUMINUM FATIGUEI ALUMINUM LITHIUM

19 ABSTRACT (Continue on reverse if necessary and identify by block number)

A 2091 plate and 8090-71651 extrusion were evaluated in this study and compared with 2024. Tensile, compression,axial fatigue, and fracture toughness tests were performed on 2091. Tensile tests were performed on 8090

Alloy 2091-T3 has a higher tensile strength, yield strength, percent elongation and modulus than 2024-T3. Alloy8090-T651 exhibits a higher tensile strength and yield strength than tha of 2024-T6. However. 8090 does not exhibitany improvement in ductility or stiffness to that of 2024-TB.

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NADC 88097-60

TABLE OF CONTENTS

List of Tables .............................................................. ii

List of Figures .............................................................. ii

Introduction .............................................................. 1

Experimental Procedure .................................................... ... 2

Results and Discussion .......................................................... 4

Conclusions .............................................................. 7

Recommendations .............................................................. 8

References .............................................................. 9

Acoesston For

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NADC 88097-60

LIST OF TABLES

Table

I Aloy Com positions ...... . .............................................. 10II Tensile Properties: 2091 and 2024 ......................................... 11III Tensile Properties: 8090 and 2024 ......................................... 12IV Compressive Strength: 2091 and 2024 ...................................... 13V Fracture Toughness Values 2091 .......................................... 14

NADC 88097-60

LIST OF FIGURES

Figure

1 Macrophotograph of 2091 plate and 8090 extrusion ........................... 152 Grain structure of 2091 plate .............................................. 163 Grain structure of 8090 extrusion .......................................... 174 Compression Samples 2091, longitudinal and transverse ....................... 185 Fatigue data, 2091 ...................................................... 196 Macrophotograph Compact tension sample, 2091 ............................. 207 Fracture Surfaces 2091, longitudinal and transverse ........................... 218 Fracture Surfaces 8090, longitudinal ........................................ 23

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INTRODUCTION

This investigation was performed as part of an ongoing evaluation program by the Naval AirDevelopment Center on advanced aluminum-lithium alloys. The Navy has considerable interest inaluminum-lithium alloys for aerospace applications. It is well known that the addition of lithium toaluminum decreases the density and increases the modulus of aluminum alloys.1 Due to theseproperties there may be significant weight savings achieved by using aluminum-lithium alloys in aircraftstructure,-::6 The objective of this study was to evaluate the mechanical properties of two aluminum-lithium alloys,

2091 -T351 and 8090-T651. Both of these alloys are being considered as medium strengthreplacements for 2024. 8090 is desirable because it exhibits a lower density with improved staticstrength properties than those of 2024. Direct replacement of 2024 with 8090 would result in a 10%weight reduction because of the lower density.2 2091 is attractive because it exhibits a lower densitywith improved strength, toughness and ductility than that of 2024. -

/ ' j"

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EXPERIMENTAL PROCEDURE

Alloy Information

The alloys examined in this investigation were made by Cegedur-Pechiney, France. They wereprovided to the Naval Air Development Center through a cooperative test program with Air Force WrightAeronautical Laboratories. The product forms included alloy 2091, 0.42 inch thick plate and alloy 8090,tee-extrusion (31n X 2.5in X 0.19in). The 2091 was received in the T351 condition. This is a mediumstrength condition which involves solution heat treatment, cold work and natural aging. The 8090 wasreceived in a the T651 condition. This is a peak aged condition which involves solution heat treatmentand artificial aging.

Microsturctura Examination

The microstructures were examined optically using a Nikon Epiphot-TME metallograph. Kellers etchwas used to bring out the grain structure. Alloy 2091 was examined in the longitudinal, transverse andshort transverse directions. Alloy 8090 was examined in the longitudinal direction.

Mechanical Properties

Tensile Tests:

The 2091 plate was machined into standard 0.252" diameter tensile specimens (longitudinal andtransverse). The 8090 extrusion was machined into flat longitudinal tensile specimens. The tensile testswere performed according to ASTM specification B557 on an MTS closed loop servohydraulic machine.Strain rates of 4.OX10"4/sec and 3.OX10"4/sec were used for alloys 8090 and 2091, respectively.

Compression Tests:

The 2091 plate was machined into compression specimens (longitudinal and transverse). Thecompression tests were performed according to ASTM specification E9 on an MTS system. A strain rateof 5XI0-4/sec was used.

Fatigue Tests:

Alloy 2091 was machined into axial fatigue specimens (Kt=1) from the longitudinal direction. Thefatigue tests were performed according to ASTM specification E466. They were run in tension-tensionon a Krause fatigue machine using a ratio of minimum to maximum load of 0.1 (R=0.1).

Fracture Toughness Tests:

Alloy 2091 was machined into compact tension specimens from the LT/L direction. The fracturetoughness tests were performed according to ASTM specification E399. These tests were ran on anMTS machine using a loading rate of 1.01 MPa m1/2/sec. The fracture toughness specimens were not avalid geometry for calculating Kic values. Also, Pm/Pq was greater than 1.1. Thus, Kq values andstrength ratios were calculated.

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FractouraDhy

The fracture surfaces of the 2091 and 8090 tensile samples and the exterior surfaces of the 2091compression samples were examined with an Amray SEM model 1000B. Macrophotographs of thefracture toughness samples were taken with a Polaroid MP-3 Land camera.

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NADC 88097-60

RESULTS AND DISCUSSION

Alloy Information

Macrophotographs of the as received material are shown in Figure 1. The material of both productforms, the plate and the extrusion, was uniform in appearance. There were not any significant flaws orporosity evident.

The compositions from the wet chemical analysis are given in Table I. Alloy 2091 contained arelatively high amount of lithium. However, both alloys were within the standard composition limits forthe alloy designations.

Microstructural Examination

The microstructure of the 2091 -T351 plate is shown in Figure 2. The plate displayed a grainstructure in which the longest dimension of the grains was aligned with the rolling direction of the plate.The average grain size was 50pm X 700pm X 1 O0pm. The grain structure of the 8090-T651 extrusionis shown in Figure 3. The 8090 extrusion had a smaller grain size and a different shape than that of the2091 plate. The average grain size of the 8090 extrusion was 6pm X 15pm X 15pm. The smallest graindimension was in the short transverse direction.

Mechanical Properties

Tensile Properties:

The 2091 -T351 plate (Table II) displayed higher tensile strength (T.S.), yield strength (Y.S.), Young'smodulus (E) and percent elongation (%e) compared with 2024-T351 plate for both testing directions.Specifically, the tensile strength of 2091 was 22 MPa higher than that of 2024 in the longitudinaldirection and 13 MPa higher in the transverse direction. The yield strength for 2091 was 41 MPa higherin the longitudinal direction and 43 MPa higher in the transverse direction. The Young's modulus was 2GPa higher in the longitudinal direction and 3 GPa higher in the transverse direction.

As expected in a rolled plate, due to the elongated grain structure, alloy strength is greatest in thelongitudinal direction. However, the plate did not exhibit anisotropy with respect to the Young's modulus.The modulus was equivalent for both testing directions.

The percent elongation for alloy 2091 was 8 percent higher in both testing directions over 2024.Usually aluminum-lithium alloys exhibit low ductility.3 Tne presence of a shearable precipitate, such asA3Li, promotes poor ductility.4 The improved percent elongation in 2091 may be due to the presence ofother alloying additions.5

Tensile properties for 8090-T651 (Table Ill) were determined only in the longitudinal extrusiondirection due to the geometry of the extrusion. The tensile strength and yield strength of 8090-T651extrusion were higher than that of alloy 2024-T6 plate. Standards for 2024-T6 extrusion were notavailable, therefore, comparison was made with 2024-T6 plate. The tensile strength of 8090-T651extrusion was 43 MPa higher than that of 2024-T6 plate. The yield strength of 8090 was 111 MPahigher. The percent elongation was slightly lower, and the Young's modulus was equivalent. The lowerpercent elongation, which portrays ductility, was expected. However, alloy 8090 was expected to have ahigher modulus The low modulus which was measured may be due to the presence of other precipitatephases such as T2 (Al6Cu Li3) or ALi.6

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As expected, the static tensile properties of the 8090-T651 extrusion exceeded those of 8090-T651plate material which are reported in the literature.7 Typically static tensile properties from an extrusionexceed those of plate material. The short transverse (ST) properties of 8090 were not evaluated in thisinvestigation due to the geometry of the extrusion. However, they may be of concern. The average shorttransverse properties of an 8090 forging have been reported in the literature.5 The ST tensile strengthwas reported as 387 MPa. The ST yield strength was 322 MPa, and the ST percent elongation was 2%.These properties are reasons for concern when considering alloy 8090 forging, extrusion, or plate forapplications where the short transverse properties are critical.

Compression Properties:

The compressive strength results are given in Table IV. The compressive yield strength of2091 -T351 is higher than that of 2024-T351. It is 29 MPa higher in the longitudinal direction and 20MPa higher in the transverse direction. The ultimate compressive strengths for 2091 are 759 MPa and744 MPa in the longitudinal and transverse directions, respectively. The compressive yield strength for2091 was less than half of the ultimate compressive strength. Yielding occurred at a lower strength levelin compression than in tension.

The percent compression was 2 percent higher in the longitudinal testing direction than in thetransverse direction. This may be due to the geometry of the grain structure relative to the applied load.Note that the longitudinal sample exhibits bending flow lines parallel to the loading axis. The transversesample does not. (See Figure 4). This is indicative of the grains undergoing micro-buckling in thelongitudinal direction. The longitudinal sample yielded more before failure. The fracture surfaces from thecompression samples occurred on a 45 degree angle to the loading direction in both samples indicatinga shear type fracture.

Fatigue Properties:

The results of the axial fatigue tests of 2091 -T351 are shown in figure 5. The threshold limit wasapproximately 32.5 ksi. After 1 e cycles at a maximum stress of 32.5 ksi failure did not occur. Fatiguedata for 2024-T351 plate was not available for these identical test conditions. However, MIL-HDBK5data for 2024-T3 sheet were available. 2024 sheet run with Kt=.02 shows lower fatigue strength. 2024sheet run with Kt=.4 shows a higher fatigue strength. The fatigue data for 2091 plate, Kt=0.1, fallsbetween these two curves. This is to be expected for 2024 sheet with a Kt=0.1. Fatigue behavior ofalloy 8090 was not examined in this study. Valid test specimens could not be machined from theextrusion. However, there is concern about rapid fatigue crack growth behavior of 8090 plate expressedin the literature.

9

Fracture Toughness Results:

Fracture toughness tests were run for alloy 2091 -T351. The fracture toughness samples yielded

before failure. A fracture surface which typified the specimens is shown in Figure 6. Evidence of yieldingis apparent from the jagged nature of the fracture surface. The specimen geometry allowed forcalculation of Kq values. The average Kq value was 45 MPa m 1 2 . The strength ratio, which is a ratio ofmaximum strength to yield strength was an average of 1.2. The pertinent variables for the fracturetoughness validity are given in Table V. Fracture toughness values were not determined for 8090 in thisstudy. The available extrusion was too thin. This sample thickness would experience predominantlyplane stress upon loading. However, the fracture toughness for 8090-T651 plate has been reported tobe comparable to 2024-T651.6 It is expected that the value for the extruded material would be at leastequivalent.

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Fractooraph,

Fracture surfaces of alloy 2091 were obtained using a scanning electron microscope. Fracturesurfaces from longitudinal and transverse tensile specimens, are shown in figures 7a & 7b. Atransgranular 450 shear fracture occurred in both testing directions. The surfaces display shear dimplesand slip. Coherent precipitates such as delta prime (AJ3Li) are shearable.4 Once shearing has begun ona particular glide plane, deformation on that plane is favored, and thus localization of slip occurs. Planarslip is res,3onsible for low ductility in aluminum-lithium alloys.3

The fracture surfaces are slightly different for the different testing directions. (Figure 7). Both surfacesdisplay transgranular fracture. However, in the transverse direction, cleavage along the grain boundariesis more pronounced. This is due to the differences in grain structure.

The fracture surfaces for alloy 8090 longitudinal tensile specimen, are shown in Figure 8. Thefracture surface was 45 degree to the loading axis in 8090 also, due to the shearable nature of the deltaprime. Intergranular fracture is evident. This is probably due to grain boundary precipitation during agingto the T6 condition.

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CONCLUSIONS

1. Alloy 2091-T351 has higher tensile strength than 2024-T351, and it exhibits an improved yieldstrength, percent elongation and modulus. Alloy 2091 -T351 exhibits comparable fatigue life to 2024-T3.

2. Alloy 8090-T651 exhibits higher tensile strength and yield strength than alloy 2024-T6. However,8090 exhibits lower ductility and no improvement in stiffness compared to 2024 in the T6 condition.

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RECOMMENDATIONS

Alloy 2091 -T351 is competitive with alloy 2024-T351 for aerospace applications. The advantages ofalloy 2091 over 2024 are a lower density with improved strength, ductility and stiffness. Furtherevaluation is needed for applications where the short transverse properties and fracture toughness arecritical.

Alloy 8090-T651 extrusion has improved tensile properties to alloy 2024-T6 plate. The advantages of8090 over 2024 are a lower density with improved static strength. However, data in the current literatureindicates that alloy 8090 is not appropriate for applications where the short transverse properties orfatigue crack growth rates are critical. Further evaluation is recommended for applications where theshort transverse properties and/or fatigue crack growth rates are critical.

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NADC 88097-60

REFERENCES

1. K.K. Sankaran and N.J. Grant, Aluminum Lithium Alloys, edited by EA Starke Jr. and T.H. SandersJr. (TMS-AIME Warrendale PA 1981) p.205

2. R. Grimes, A.J. Cornish, W.S. Miller and M.A. Reynolds, Metals and Materials, Vol. 1, pp.357-363(1985)

3. B. Noble, S.J. Harris and K. Dinsdale, "Yield Characteristics of Al-Li Alloys", Metal Science, 16(1982) pp.425-430

4. M. Tamura, T. Mori, and T. Nakamura, "Precipitation of A3Li from An Al-3% Li Alloy and SomeProperties of Al31i", J. Japan Institute of Metals, 34 (1970) pp.919-925.

5. K. Dinsdale, S.J. Harris and B. Noble, "Relationship Between Microstructure and MechanicalProperties in Al-Li-Mg Alloys", Aluminum-Lithium Alloys II, edited by T.H. Sanders, Jr. and E.A.Starke, Jr., (TMS - AIME Warrendale, PA 1983) p.101

6. M.E. O'Dowd, W. Ruch, and E.A. Starke, Jr., "Dependence of Elastic Modulus on Microstructure in2090-Type Alloys", Journal of Physics, Colloque C3, supplement number 9, Volume 48, September1987, pp.C3-5655.

7. M.R. James, Scripta Met. Vol.21, pp. 783-788, (1987)

8. D. Webster, R. Kirkbride, "Mechanical Properties and Microstructure of Al-Li-Cu-Mg-Zr DieForgings", Met. Trans. A., Vol.17A, pp.2007, Nov.(1986).

9. W.G.J. 't Hart, H.J. Kolkman, L. Schra and R.J.H. Wanhill, "Advanced Aluminum Alloy Plate Materialsfor Damage Tolerant Aircraft Structures", National Aerospace Lab, Amsterdam (Netherlands),PB87-156147, Nov. (1985)

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TABLE I CHEMICAL COMPOSITIONS (wt%)

ALLOY Li Cu Mg Si Fe Hn Zr Al.

# 2091 1.7-2.3 1.8-2.5 1.1 1.9 .2 .3 .10 .04-.16 balance

+ 2091 2.30 2.00 1.50 .05 .015 .10 .12 balance

1 8090 2.1 2.7 1.1 1.6 .08-1.4 .10 .15 .05 .04-.16 balance

* 8090 2.41 1.20 .70 .04 .016 .10 .15 balance

* 2024 - 4.40 1.50 - .200 .60 - balance

* Wet Chemical Analysis

* "ASH Databook", 1979 Metal Progress vol.116. no.1.

# Acceptable composition range, Aluminum Association 1988

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TABLE 11. TENSILE DATA2091, 0.42 inch plate2024, 0.5 inch plate

ALLOY 2091-T351 2024-T351. 2091-T351 2024-T351.

Testing

Direction Longitudinal Longitudinal Transverse Transverse

Y.S. (MPa) 387 345 346 303

U.T.S (MPa) 470 448 461 448

% ELONGATION 16 8 16 8

MODULUS, E (GPa) 76 74 77 74

* IL-Handbook V data, p.3-69, I June 1987

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TABLE III.TENSILE DATA8090 and 2024

ALLOY 8090-T651 *2024-.T42 *2424-462

Product form Extrusion Extrusion Plate

Testing

Direction Longitudinal Longitudinal Longitudinal

Y. S. (MPa) 488 262 345

U.T.S. (MPa) 552 393 441

Z Elongation 3 12 5

Modulus, E (GPa) 72 74 72

* IL-Handbook V data, p.3-70,3-71, 3-79, 1 June 1987.

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TABLE IVCOMPRESSION TEST RESULTS

Compression Compression

ALLOY Y.S.(MPa) T.S.(MPa) %Compression

2091-T351

Longitudinal 311 759 30

Transverse 344 744 28

*2024-T351

Longitudinal 283

Transverse 324

* MIL-Handbook V data, p.3-69. 1 June 1987.

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TABLE V FRACTURE TOUGHNESS

ALLOY Pm/Pq Rsc Kq (MPa m )

2091-T351

LT 1.17 1.2 44.6

P - Maximum Load sustainedm

P - Calculated offset loadq

R sc- Strength Ratio, R ino(2P m(2W+a))/(BCW-a)2 )s

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NADC 88097-60

a)

b)

2cm

Figure 1. Macrophotographs a) 2091 Plateb) 8090 Extrusion

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Figure 2. Grain structure of 2091 plateEtched with Keller's reagent

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O.1mm

Figure 3. Grain Structure 8090, Extrusion

Etched with Keller's Reagent

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a)

5 cm

b)

. 5 cm'

Figure 4. Compression Samples 2091a) Longitudinalb) Transverse

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42j0

CL-

00

:3 0

~0 u

U

0)0

W') 0 In 0 U) a LO 0 U')

in (D Ln n 1* qt n nN

Figure 5 Fatigue Data . Alloy 2091, 0.42 inplate

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Figure 6 Macrophotograph Compact Tension, Sample 2091

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7a)

A~~~ ip i iS S S

Figure 7 Fracture Surfaces, 2091a) Longitudinalb) Transverse

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7b)

tid,

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6

Figure 8 Fracture Surfaces, 8090

Longi tudtial

23

mmum atm ol Ia!!! I I I I

NADC 88097-60

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